[sam] Read unique ID correctly for SAMG55

ext/hathach/tusb_port.cpp.in

@@ -12,6 +12,7 @@
 
 #include <modm/platform/device.hpp>
 #include <modm/architecture/interface/interrupt.hpp>
+#include <modm/architecture/interface/atomic_lock.hpp>
 #include "tusb.h"
 
 %% for irq in irqs | sort
@@ -21,37 +22,68 @@ MODM_ISR({{irq}})
 }
 %% endfor
 
+%% if "samg55" in target.string
+
+// Read unique ID using the flash controller
+// This function is placed in RAM, because code cannot be fetched from flash
+// while reading the unique ID.
+modm_ramcode void __attribute__ ((noinline)) read_efc_uid(uint32_t *uid)
+{
+	// Disable interrupts -- we can't fetch code
+	// This function was measured to take 660ns on a SAMG55 running at 100MHz
+	modm::atomic::Lock lck;
+
+	// Issue Start Read Unique Identifier command
+	EFC->EEFC_FCR = EEFC_FCR_FCMD_STUI | EEFC_FCR_FKEY_PASSWD;
+	// Wait for flash controller ready bit to go low
+	while(EFC->EEFC_FSR & EEFC_FSR_FRDY) {};
+
+	for(uint32_t i=0; i<4; i++) {
+		uid[i] = *(uint32_t*)(IFLASH_ADDR + i * 4);
+	}
+
+	// Issue Stop Read Unique Identifier command
+	EFC->EEFC_FCR = EEFC_FCR_FCMD_SPUI | EEFC_FCR_FKEY_PASSWD;
+	// Wait for flash controller ready bit to go high
+	while(!(EFC->EEFC_FSR & EEFC_FSR_FRDY)) {};
+}
+
+%% endif
 
 extern "C" uint8_t
 tusb_get_device_serial(uint16_t *serial_str)
 {
 	constexpr uint8_t SERIAL_BYTE_LEN = 16;
+	uint8_t raw_id[SERIAL_BYTE_LEN];
+
+%% if "samg55" in target.string
+	read_efc_uid((uint32_t*)raw_id);
+%% else
 
 	uint32_t *id_addresses[4] =
 	{
-%% if target.platform in ["stm32"]
+	%% if target.platform in ["stm32"]
 		((uint32_t *) UID_BASE), ((uint32_t *) UID_BASE) + 1,
 		((uint32_t *) UID_BASE) + 2, ((uint32_t *) UID_BASE) + 3
-%% elif target.platform in ["sam"]
-	%% if "samd51" in target.string
+	%% elif target.platform in ["sam"]
+		%% if "samd51" in target.string
 		(uint32_t *)0x008061FC, (uint32_t *)0x00806010,
 		(uint32_t *)0x00806014, (uint32_t *)0x00806018
-	%% else
+		%% else
 		(uint32_t *)0x0080A00C, (uint32_t *)0x0080A040,
 		(uint32_t *)0x0080A044, (uint32_t *)0x0080A048
-	%% endif
-%% elif target.platform in ["rp"]
+		%% endif
+	%% elif target.platform in ["rp"]
 		/* sysinfo CHIP_ID and GIT_REV */
 		((uint32_t *)0x40000000),((uint32_t *)0x40000040),
 		((uint32_t *)0x40000000),((uint32_t *)0x40000040),
-%% endif
+	%% endif
 	};
 
-	uint8_t raw_id[SERIAL_BYTE_LEN];
-
 	for (int i = 0; i < 4; i++)
 		for (int k = 0; k < 4; k++)
 			raw_id[4 * i + (3 - k)] = (*(id_addresses[i]) >> k * 8) & 0xff;
+%% endif
 
 	const auto fn_nibble = [](uint8_t nibble)
 	{